1. Field of the Invention
The present invention relates to an inline optical fiber attenuator and, more particularly, to an inline optical fiber attenuator formed with only two fiber components, a birefringent polarization-preserving fiber and a single-polarization fiber, where the degree of attenuation is adjusted merely by changing the birefringence in the polarization-preserving fiber.
2. Description of the Prior Art
Optical fiber attenuators find a wide variety of uses and will become more prevalent as more and more electrical communication systems are replaced with optical fiber communication systems. For example, fiber attenuators are utilized in testing manufactured optical fiber cable, comparing signals propagating along various fibers. In the laboratory environment, attenuators may be employed in experimental apparatus to control, for example, a laser beam intensity where a predetermined intensity is so required. Further, optical fiber attenuators may be utilized with a feedback control system to stabilize the output therefrom, correcting for coupling losses and drift in the output beam, thus stabilizing the intensity of the output beam.
One prior art attenuation arrangement is disclosed in U.S. Pat. No. 4,261,640 issued to W. C. Stankos et al. on Apr. 14, 1981 which relates to an inline optic attenuator comprising a light absorbing optical disc and two fiber sections, where the entire amount of desired attenuation is accomplished by exposing the photographic film forming the optical disc to a degree sufficient to equal the desired attenuation of the simulated length of optical fiber. In operation, the disc must be quite thin since the light beam will tend to disperse in the disc as it passes between the ends of the fibers attached thereto. If the dispersion is great enough, it will cause multiple path lengths and possibly multimode pickup in the receiving optic fiber, thereby creating pulse distortion in the optical path.
An alternative prior art arrangement is disclosed in U.S. Pat. No. 4,257,671 issued to A. Barbaudy et al. on Mar. 24, 1981. In this arrangement, the attenuator comprises an attenuator element and two plane, graded index lenses in alignment with each other and separated by the attenuator element. The attenuator element is formed of an organic or inorganic medium which is transparent to the wavelength of the light rays and which is doped with one or more substances which have an absorption band at the wavelength employed. The use of the graded index lenses eliminates many of the spurious reflections associated with other prior art arrangements.
An optical fiber connector which may be employed as an inline attenuator is disclosed in U.S. Pat. No. 4,145,110 issued to O. Szentesi on Mar. 20, 1979. Here, the attenuator comprises a connecting element with a common bore of size to accept with close sliding fit two optical fibers in an end-to-end relationship. Generally, one fiber is permanently held in the connecting member, as by crimping or bonding, and the other fiber is moved by a linear positioner which moves the fiber back and forth axially to increase and decrease the end separation of the two fibers, thus increasing and decreasing the attenuation. Such an arrangement, however, is of limited ability in fine-tuning the attenuation in association with the mechanical nature of the adjustment procedure and is extremely sensitive to small separations, on the order of a few wavelengths.
There remains a need in the field of optical fiber attenuators, however, for an inline optical fiber attenuator which is completely contained within the fiber itself, that is, requiring no external bulk components, and which is capable of being easily adjusted for the varying amounts of attenuation desired.